This renewal application is an extension of previous studies aimed at elucidating the control of the cell cycle using cell cycle-specific temperature-sensitive mutants. Cell cycle-specific ts mutants are operationally defined here as cell lines that grow normally at the permissive temperature, but at the nonpermissive temperature arrest in a specific phase of the cell cycle. Using these ts mutants and techniques of cell hybridization, we intend to ask a number of questions concerning the transition of cells in culture from the resting to the growing stage. Among these questions are: 1) Is the transition from the resting to the growing stage nucleus dependent? 2) What is the role of RNA polymerase II in the G1 phase of the cell cycle? 3) What is the role of RNA polymerase I in the Go to S transition, and is it dependent on an active RNA polymerase II? 4) Are certain events of the G1 period orderly arranged or not, as for instance phosphorylation of H1? Finally, we would like to develop some new techniques for the introduction of biologically active macromolecules into cells while preserving their viability. These studies should throw some light on some of the mechanisms that control the transition from the resting to the growing stage in cells in culture, as well as on the events in the G1 period that is apparently the critical period for cell growth. In turn, an elucidation of cell cycle controls may be helpful in our understanding of the growth of abnormal cells, like transformed cells.